Pokemon Ruby, Sapphire, Emerald

How do I get Kingdras?

Games Guru: Seadra, the unevolved form of Kingdra, is a catchable species in the Ruby, Sapphire, Emerald, Leaf Green and Fire Red versions. Kingdra is not. To own Kingdra, you must catch Seadra, train it to level 32, give it a dragon scale as a hold item, and trade. It evolves upon being traded the way Machamps and Golems evolve upon being traded.

How do you evolve Golbats into Crobat? Or is there another way to do it?